The present invention relates generally to lasers requiring high voltages, and more specifically to pulse forming networks which are formed from coaxial cables. Coaxial cables have been used as pulse forming networks (PFN) since before 1940, and were the basis for the derivation of the canonical lumped parameter networks during WW II. The pulse fidelity from cables is unequaled but there are some limitations and disadvantages inherent with their use. The most significant limitation is the length of cable required, typically about 90 meters per microsecond. Another limitation is the practical range of characteristic impedance being about 10 to 100 ohms, but this at least is within the range of many PFN applications. Both of these limitations are somewhat overcome by the lumped parameter PFN, usually with an acceptable degradation in pulse fidelity. However, when scaling to very high voltage and energy, problems arise with the lumped parameter PFN which begin to put them at a disadvantage with respect to coaxial lines. Specifically, stray inductance and capacitance of the lumped parameter PFN causes serious degradation of pulse fidelity. A coaxial cable is a distributed parameter device and the stray problems with scaling does not exist. Another disadvantage with conventional cable PFN systems is the size and volume required. When lumped parameter devices such as conventional PFN's, Blumlein's Marx generators, etc., are scaled up, they require large oil filled tanks which are also very costly. The actual cables themselves are self contained regardless of the size, and can be orders of magnitude smaller in the overall space required and the total cost.
The task of providing a pulse forming device using coaxial cable technology without high space requirements, and without waveform degradation due to stray capacitance and inductance is alleviated, to some degree, by the following U.S. Patents, the disclosures of which are incorporated by reference:
U.S. Pat. No. 3,463,992 issued to Solberg;
U.S. Pat. No. 3,845,322 issued to Aslin;
U.S. Pat. No. 4,005,314 issued to Zinn;
U.S. Pat. No. 4,484,085 issued to Fallier, Jr. et al; and
U.S. Pat. No. 4,549,091 issued to Fahlen et al.
Zinn discloses a device for achieving a narrow high voltage output pulse having a rise time essentially limited only by the rise time of the switching means. It comprises a voltage source connected through a semiconductor switch to an energy storage device. This device uses a plurality of capacitors fabricated similarly to a conventional coaxial cable, but includes a plurality of conductive elements with insulating means disposed between each element. Holdoff semiconductor diodes are connected to the coaxial elements and a load is connected between the outer element and ground. The capacitors of the reference are charged in parallel from the single power source and discharged in series through a second current path which includes the common load.
Soldberg discloses a capacitive storage system which employs capacitors having different time constants and charged with a voltage magnitude and polarity different for each capacitor. This patent discloses two capacitors in series charged separately to unequal voltages of opposite polarity for long term energy storage.
Aslin shows a Marx type pulse generator. Fahlen et al discuss a pulsed electrical power circuit for high repetition rate gas lasers. In Fallier, Jr. et al a pulse generator is formed by overlapping conductive spiral strips separated by insulating strips.
Pulse forming networks composed of coaxial cables which use two conductors provide comparatively low energy storage utilization, have high cost and space requirements and produce waveform degradation due to stray capacitance and inductance. There remains a need in the art to provide a pulse forming device which reduces these problems. The present invention is intended to satisfy that need.